In many data acquisition or data processing environments data is routed to a measurement device, or between functional elements within an apparatus, as a serial stream of bits. The electrical signals comprising such a data path may be single ended (data is one conductor measured with respect to a system ground) or balanced (differential signals on a pair of conductors). The serial stream of data may be accompanied by a clock signal on a separate conductor, or it may be self-clocking. Whatever the electrical nature of the serial data and its clock, they are susceptible of measurement by oscilloscopes and various specialized types of analyzers. Oscillographic presentations are generally waveforms, while analyzers may present captured data either as waveforms or as symbols arranged in tabular form. In either case it is generally necessary or desirable to capture and display only conditions surrounding an event of interest. The detection of such events is called triggering. A serial trigger is a functional element that is responsive to the serial stream of data and its associated clock by detecting the occurrence of a stated pattern in the data and producing a transition in a trigger signal.
To define such a pattern is to also specify the number of consecutive bits that the pattern is to occupy. We shall term that number N, and expect N to be at least one, and no larger than some reasonable (although still arbitrary) limit. We shall say that N defines a word length for a trigger word. Since it is often convenient for all words to be of the same length, it is fair to equate serial word length with the corresponding concept of word width that occurs in parallel architectures. But whereas each instance of a clock signal in a parallel environment can indicate a new word, in a serial environment there must be N cycles of the clock. This raises the question of which such clock cycle is associated with the start of a new word. A signal that indicates the start of a new word is called a framing signal, or sometimes a framing pulse. In a situation where there is no larger construct formed of individual words, then each Nth clock cycle could serve as a framing pulse, once the progression has been started, but this still requires that an initial clock cycle be identified as synchronous with the start of the word. We shall term such an initial/Nth clock cycle a framing clock cycle. It is possible that a separate signal indicates the framing pulse. Another circumstance is that there are times when there is no clock signal for at least a minimum length of time. Then the first clock cycle thereafter ought to be a framing clock cycle.
It is also possible to have multiple words between framing pulses. In such a circumstance we would say that a framing pulse identifies the start of a record, and that a record comprises a number of words.
Given all these possibilities, it would be useful if a serial trigger were able to accept an N-bit trigger pattern definition and an M-bit delay past the framing pulse, until after which delay the serial trigger ignores any pattern matches between the incoming data and the pattern. M might be arbitrary, but is most likely to be zero, N, 2N, etc. Such an arrangement for a serial trigger allows for its use with either the case where a frame is a single word or the case where a frame is a record of two or more words. It allows the trigger word to be positioned at any word within a record. Further flexibility is gained by allowing selected bits within the trigger pattern to be defined as "don't care" bits.
The above remarks, while true, may apply most particularly to communication channels between systems. Internally, many serial systems operate asynchronously with respect to the start of words. That is, the absence of a clock signal for the above mentioned minimum length of time is an implied framing pulse, or is at least a start of word indicator. (This idea can be realized with and without a separate framing pulse.) For example, a system might issue an instruction to load a certain register from a particular bus. Then the next N-many clock cycles on that bus are associated with the N bits on that bus that are to be serially clocked into the register. Another more general case might be any serially programmed mechanism. Consider an oscilloscope or specialized analyzer connected to a serial data path associated with a system. The desired measurement requires the use of a serial trigger, but no framing pulse or other explicit start-of-word signal is readily available. What to do?